How to Flip a Video Clip
Compositor:
This can be done with the Flip Node within the compositor: Add> Distort> Flip.
The node should look like this:



Video-Sequence Editor:
This can be easliy don with the flip options in your Properties Panel N. The options should look like this:


For your other question I'm pretty sure you need to re-open your video, essentially your not editing the video but making a mask which can be accesed with the Mask Node so it doesn't really matter as long your using the same footage.